1. Field of the Invention
The present invention relates to a rotating electric machine.
2. Discussion of the Background
Examples of rotating electric machines are synchronous machines, normal asynchronous machines as well as double-fed machines, alternating current machines, applications in asynchronous static current converter cascades, outerpole machines and synchronous flux machines.
The machine according to the invention is intended in the first place as a generator in a power station for generating electric power.
In the present application the terms xe2x80x9cradialxe2x80x9d, xe2x80x9caxialxe2x80x9d, xe2x80x9ctangentialxe2x80x9d and xe2x80x9cperipheralxe2x80x9d constitute indications of direction defined in relation to the rotor of the machine unless expressly stated otherwise.
For vertical rotating electric machines with salient poles, known as Salient Pole Machines, coil supports are conventionally used between the different poles. The reason for this is that the rotor poles comprise pole pieces provided with a winding so that solenoids (magnet coils) are formed. Since the pole has a certain width, the long sides of the coil will not be situated in an axial plane running through the centre of the rotor, i.e. an axial plane through the long sides of the coil will be displaced parallel to an axial plane through the radial centre line of the corresponding pole. Part of the centrifugal force arising when the rotor rotates will therefore influence the long sides with a tangential force in the direction of the periphery. This force will endeavour to press the metal in the coil winding away from contact with the pole piece. The long sides of the coil thus run the risk of being bent out and, in the worst case, coming into contact with each other, which may give rise to powerful short circuits which in turn cause disturbing magnetic imbalances.
In order to prevent short circuits of the type described above, therefore, special coil supports are fitted in the pole gap, i.e. the gap or space between two adjacent poles, when deemed necessary. The task of these coil supports is thus to keep the coil windings in place. Coil supports are especially common in coils in high-speed machines.
Conventional coil supports usually consist of one or more whole-block supports, i.e. solid, substantially V-shaped supports, arranged a suitable axial distance from each other in each pole gap. Since it is desirable for the coil support to support the entire solenoid winding, each coil support will substantially block the entire area between two adjacent poles, i.e. the pole gap. The small space between two rotor poles which is not blocked by the coil support is used for ventilation so that cooling air can flow axially to cool the poles and later also the stator, depending on the type of rotating machine and the type of ventilation principle utilized. The spaces referred to are essentially the small space available near the air gap, between the rotor poles and the stator, and the equally limited space between the rotor windings and the rotor itself.
The types of ventilation which are relevant are the so called Midi-concept, axial ventilation and radial ventilation. In the Midi-concept the rotor body is used as a fan and the air passes axially through the pole gap from one side of the machine. In the case of axial ventilation axial fans are mounted on the rotor, usually two fans, one on the upper and one on the lower side. The air effects cooling of the poles and is transported axially to radial openings in the stator where it also has a cooling effect. In the absence of radial openings in the stator, however, the alternative using two axial fans is not possible. One-way axial ventilation or radial ventilation (air flowing radially through the rotor) can however manage the ventilation without radial openings in the stator core. However, radial ventilation is not possible with small rotor diameters.
It is obvious that the cooling air, or possibly some other type of gas used for cooling, able to pass the coil supports is in many cases insufficient to achieve satisfactory cooling of the machine.
The flow of cooling air permitted to pass through the pole gap is also limited by the maximum rise in temperature permissible for the air and also by the air speed having to be restricted. The latter is because, when the air is conducted through the pole gaps, it is accelerated to a speed approaching the peripheral speed of the rotor. This results in an increase in the kinetic energy of the air which is entirely lost, however, when the air leaves the machine. A retarding moment is thus caused, and consequently a power loss, which is directly reflected in the efficiency of the machine. Since these ventilation losses are directly proportional to the air flow, it is therefore desirable to have as little air resistance as possible.
According to a known device, in which an attempt is made to solve the problem of insufficient ventilation, the coil supports have been provided with apertures in order to increase the available area for the flow of cooling air. Unfortunately, these coil supports in many cases still fail to provide satisfactory ventilation. Furthermore, the coil supports force the air away from the coils which results in at least some of the air flow instead flowing closer to the stator and thus passing too far away from the coils to be of any use.
From the document U.S. Pat. No. 3,740,600 is previously known a coil support or brace which supports only part of the coil windings, namely those windings which are closest to the rotor piece. The purpose of the brace is to prevent separation of the coils from the poles by the tangential component of centrifugal force acting on the coils. The reason that the brace is designed to support only the innermost windings is that these windings, in the disclosed configuration of the poles, are particularly subjected said forces. Similar devices are also known from U.S. Pat. No. 5,036,238, GB-A-2 022 327 and DE-A-25 20 511.
Through U.S. Pat. No. 5,036,165 is known a conductor, in which the insulation is provided with an inner and an outer layer of semiconducting pyrolized glass fibre. It is also known to provide conductors in a dynamo-electric machine with such an insulation, as described in U.S. Pat. No. 5,066,881 for instance, where a semiconducting pyrolized glass fibre layer is in contact with the two parallel bars forming the conductor, and the insulation in the stator slots is surrounded by an outer layer of semiconducting pyrolized glass fibre. The pyrolized glass fibre material is described as suitable since it retains its resistivity even after the impregnation treatment.
The object of the present invention is to provide a rotating electric machine designed to eliminate the above-mentioned problems and whereby satisfactory ventilation is obtained in an efficient and simple manner.
This object is achieved by a machine of the type described herein.
Accordingly, the coil supports are arranged in pairs, and each coil support in a pair is shaped so that it supports only a part of the solenoid windings of the two adjacent poles. To continue, two coil supports in a pair are arranged axially and radially displaced in relation to each other to allow a cooling gas to flow between the coil supports. Through this arrangement the advantage is gained that the area of the flow path for the air through the pole gaps is increased and the air resistance is decreased. A larger amount of air (or other gas) can thus pass at the same flow rate, and a higher degree of cooling is obtained. Alternatively, the same amount of air may be allowed to pass at a reduced rate, whereby said power losses are reduced and the efficiency of the machine increases. This in turn offers the advantage of being able to use only one fan instead of two, as is normal with axial ventilation. Satisfactory ventilation of the coil-end package beneath the stator is also made possible with this single fan. As compared to known devices for supporting only part of the windings, the present invention has the considerable advantage that all of the windings will be supported, thus resulting in a more reliable and altogether safer machine.
Another advantage is that, if radial ventilation (air flowing radially through the rotor) is not possible, particularly in rotors with small diameters, the present invention enables one-way axial ventilation through the entire pole gap to the lower side of the stator in combination with coil supports.
The coil supports characterized in claim 1 also have the advantage that they can be used both in rotating electric machines of conventional type and in high-voltage machines using cable, i.e. high-voltage insulating electric conductors, as stator windings. Particularly pronounced advantages are achieved when the machine according to the invention is provided with stator windings composed of high-voltage cable. The high-voltage cable lies in slots in the stator which are extra deep and vertically high, and efficient ventilation is thus particularly difficult to achieve, as well as the need for it being greater.
Consequently, in the rotating electric machine according to the invention, the windings are preferably composed of cables having solid, extruded insulation, of a type now used for power distribution, such as XLPE-cables or cables with EPR-insulation. Such a cable comprises an inner conductor composed of one or more strand parts, an inner semiconducting layer surrounding the conductor, a solid insulating layer surrounding this and an outer semiconducting layer surrounding the insulating layer. Such cables are flexible, which is an important property in this context since the technology for the device according to the invention is based primarily on winding systems in which the winding is formed from cable which is bent during assembly. The flexibility of a XLPE-cable normally corresponds to a radius of curvature of approximately 20 cm for a cable with a diameter of 30 mm, and a radius of curvature of approximately 65 cm for a cable with a diameter of 80 mm. In the present application the term xe2x80x9cflexiblexe2x80x9d is used to indicate that the winding is flexible down to a radius of curvature in the order of four times the cable diameter, preferably eight to twelve times the cable diameter.
The winding should be constructed to retain its properties even when it is bent and when it is subjected to thermal stress during operation. It is vital that the layers retain their adhesion to each other in this context. The material properties of the layers are decisive here, particularly their elasticity and relative coefficients of thermal expansion. In a XLPE-cable, for instance, the insulating layer consists of cross-linked, low-density polyethylene, and the semiconducting layers consist of polyethylene with soot and metal particles mixed in. Changes in volume as a result of temperature fluctuations are completely absorbed as changes in radius in the cable and, thanks to the comparatively slight difference between the coefficients of thermal expansion in the layers in relation to the elasticity of these materials, the radial expansion can take place without the adhesion between the layers being lost.
The material combinations stated above should be considered only as examples. Other combinations fulfilling the conditions specified and also the condition of being semiconducting, i.e. having resistivity within the range of 10xe2x88x921-10xe2x88x926 ohm-cm, e.g. 1-500 ohm-cm, or 10-200 ohm-cm, naturally also fall within the scope of the invention.
The insulating layer may consist, for example, of a solid thermoplastic material such as low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP), polybutylene (PB), polymethyl pentene (xe2x80x9cTPXxe2x80x9d), cross-linked materials such as cross-linked polyethylene (XLPE), or rubber such as ethylene propylene rubber (EPR) or silicon rubber.
The inner and outer semiconducting layers may be of the same basic material but with particles of conducting material such as soot or metal powder mixed in.
The mechanical properties of these materials, particularly their coefficients of thermal expansion, are affected relatively little by whether soot or metal powder is mixed in or notxe2x80x94at least in the proportions required to achieve the conductivity necessary according to the invention. The insulating layer and the semiconducting layers thus have substantially the same coefficients of thermal expansion.
Ethylene-vinyl-acetate copolymers/nitrile rubber (EVA/NBR), butyl graft polyethylene, ethylene-butyl-acrylate copolymers (EBA) and ethylene-ethyl-acrylate copolymers (EEA) may also constitute suitable polymers for the semiconducting layers.
Even when different types of material are used as base in the various layers, it is desirable for their coefficients of thermal expansion to be substantially the same. This is the case with the combination of the materials listed above.
The materials listed above have relatively good elasticity, with an E-modulus of E less than 500 MPa, preferably  less than 200 MPa. The elasticity is sufficient for any minor differences between the coefficients of thermal expansion for the materials in the layers to be absorbed in the radial direction of the elasticity so that no cracks appear, or any other damage, and so that the layers are not released from each other. The material in the layers is elastic, and the adhesion between the layers is at least of the same magnitude as in the weakest of the materials.
The conductivity of the two semiconducting layers is sufficient to substantially equalize the potential along each layer. The conductivity of the outer semiconducting layer is sufficiently high to enclose the electrical field within the cable, but sufficiently low not to give rise to significant losses due to currents induced in the longitudinal direction of the layer.
Thus, each of the two semiconducting layers essentially constitutes one equipotential surface, and these layers will substantially enclose the electrical field between them.
There is, of course, nothing to prevent one or more additional semiconducting layers being arranged in the insulating layer.
According to a particularly advantageous feature the stator windings are composed of high-voltage cable.
An additional characteristic is that the high-voltage cable has a diameter within the interval 20-250 mm and a conductor area within the interval 80-3000 mm2.
Further characteristics and advantages of the present invention are revealed in the remaining sub-claims.
According to another characteristic, each of the two coil supports in a pair covers substantially half the area occupied by the corresponding pole gap in the radial direction. The advantage is thus gained that the remainder of the pole-gap area, i.e. as much as the remaining 50%, is available for the air flow.
Alternatively, one of the two coil supports in a pair is arranged to support substantially half of the windings of the two adjacent poles and the other of the two coil supports is arranged to support substantially the other half of said windings.
According to yet another alternative, one of the two coil supports in a pair is arranged to support substantially half of the windings of the two adjacent poles and the other of the two coil supports is arranged to support the other half of said windings and an additional small number of windings in order to secure an overlap of supported windings. Through this alternative is achieved a particularly safe device due to the overlap of supported windings.
Finally, according to a particularly advantageous feature, the two coil supports in a pair are displaced axially by such a distance that the area in the axial direction between the two coil supports substantially corresponds to that part of the area of the pole gap in the radial direction which is not occupied by a coil support. Through this is ascertained that the area available for the flow of cooling gas in the axial direction, between each support in a pair, is substantially equal to the area available in the radial direction, i.e. that area in a pole gap where the air flow is not prevented by a pole support.
Rotating electric machines have conventionally been designed for voltages in the range 6-30 kV, and 30 kV has normally been considered to be an upper limit. This generally means that a generator must be connected to the power network via a transformer which steps up the voltage to the level of the power network, i.e. in the range of approximately 130-400 kV.
By using high-voltage insulated electric conductors, also termed high-voltage cables, with solid insulation similar to that used in cables for transmitting electric power (e.g. XLPE cables) the voltage of the machine can be increased to such levels that it can be connected directly to the power network without an intermediate transformer. The conventional transformer can thus be eliminated. However, an important condition for this is that satisfactory ventilation can be arranged, which is thus enabled by the present invention.
The present invention is in the first place intended for use with high voltages. High voltages shall be understood here to mean electric voltages in excess of 10 kV. A typical operating range for a device according to the invention may be voltages from 36 kV up to 800 kV. In the second place the invention is intended for use in the stated technical area at voltages below 36 kV.